The present invention relates to a scanning apparatus capable of scanning and reading a faint intensity of light emitted from a pattern of a sample of flat-plate shape at a high degree of sensitivity and, more particularly, to a scanning apparatus for scanning and reading, as image data, a pattern of luminescence emitted to a faint extent from a sample of flat-plate shape.
Techniques of analysis by gel electrophoresis have hitherto been extensively employed for fractionalization or analysis of structures of proteins or nucleic acids of high molecular substances of a living body. In many cases, however, the samples are to be obtained only in very small amounts by means of gel electrophoresis so that a secure and high degree of sensitivity to detection is particularly required for analysis of such samples.
Therefore, there have hitherto been applied techniques to a sample available in a very small amount, which involve labelling the sample as an object for analysis with a radioactive substance, pouring the labelled sample into a gel, subjecting the gel to electrophoresis, attaching the electrophoresed gel onto an X-ray film or the like for exposure to the sample labelled with the radioactive substance, transferring the light emitted from the radioactive substance to the X-ray film, and scanning and reading the light as an electrophoresis pattern of the sample.
It should be noted, however, that radioactive substances are very dangerous to handle and extreme care should be paid in handling and management of such radioactive substances. Therefore, recently, in order to require no use of such dangerous radioactive substances, there have been developed chemical luminescence methods which can detect an electrophoresis pattern of a sample at a high degree of sensitivity by means of chemical luminescence. Such chemical luminescence methods comprise labelling a sample with a substance such as an enzyme, admixing the sample with a luminescent substrate to cause a chemical luminescence due to a chemical reaction between the substance labelled in the sample and the luminescent substrate, and exposing the chemical luminescence emitted from the sample to a film to thereby provide a pattern of the sample.
More specifically, the results of blotting obtained by a chemical luminescence method are read by adding a luminescent substrate to a membrane to which a sample is attached, causing chemical luminescence from the sample in accordance with an amount of the sample, and exposing the chemical luminescence to a highly sensitive film, and scanning the chemical luminescence of the film. In some experiments, as chemical luminescence is very faint, a chemical luminescent substance capable of emitting chemical luminescence for a long period of time is required to be employed and a film having a high degree of sensitivity is also required to be employed. Even in such instances, ten to twenty hours are required for exposure of the sample to the highly sensitive film. After completion of the exposure, the film is then developed to provide a desired blotting image. In order to analyze the resulting image in a quantitative way, for example, it is further scanned with a scanner for general use in scanning documents and images, fetched in a personal computer, and analyzed by image processing.
In the above-described method using the highly sensitive film, a period of time to be set for exposure of a sample to the film has been determined, in many cases, by experiences and feelings of an operator on the basis of the amount of the sample used, the amount of a chemically luminescent substance to be used for the sample, temperature conditions and so on. Therefore, experimental results cannot be gained in a stable and secure manner and experiments have been forced to be carried out again in many cases. Further, such a conventional method is encountered with the problem that measurements cannot be done in a wide range due to the fact that a dynamic range should be determined by sensitivity characteristics of the film.
More recently, there has been made an attempt to solve the problems as described hereinabove, which is involved in reading the light of chemical luminescence directly as an image by means of a highly sensitive camera. In this case, in order to enable the camera to read a very small amount of the chemical luminescence, the sensitivity of the camera is required to be high enough to detect photons in a level of approximately 10.sup.4 per square centimeter per second. There are currently available two types of optical reading apparatuses that can achieve such a high degree of sensitivity; one being of such a type as amplifying optical signals with an image intensifier and then fetching an image, and the other being of such a type as improving a signal-to-noise ratio by controlling the generation of thermal noise by cooling a CCD image pick-up element.
The optical reading apparatus of the former type comprises the image intensifier of a structure in which a large number of capillaries for amplifying electrons, which are referred to as multi-channel plates, are interposed between photoelectric surfaces and the optoelectrons generated at the front photoelectric surface are caused to amplify to 10.sup.4 to 10.sup.5 times and be returned to optical signals at the rear photoelectric surface to be relayed them to a camera. The optical reading apparatus of this type, however, presents the problems that the cost of the image intensifier is very expensive and that the resolution capability of an image is restricted by the number of the multi-channel plates.
On the other hand, the optical reading apparatus of the latter type comprises a CCD image pick-up element of cooling type. A camera of this optical reading apparatus can reduce thermal noises by cooling the CCD element to minus several tens Celsius, thereby raising a signal-to-noise ratio. The CCD element is subjected to electronic cooling and the heat generated is sucked out by a medium such as water or liquid nitrogen. The CCD image pick-up element of such highly sensitive type can detect illuminance of approximately 10.sup.-8 lx and pick up an image. Further, it can gain a higher sensitivity by accumulating the light for a long period of time. The optical reading apparatus of the cooling type, however, presents the problems that a pump instrument and so on are required for circulating the cooling medium, thereby making the apparatus bigger in size, and that costs for the apparatus itself and maintenance become expensive.
Further, the problem common in the optical reading apparatuses of camera type exists in the fact that the number of pixels of the image pick-up element is as small as 512 by 512, in usual cases, and as much as 1,024 by 1,024, in larger cases. When an image of a pattern of a sample is to be scanned and read by the image pick-up element having such a small number of pixels, however, distortion is likely to be introduced in the optical system, and enlarging or reducing of the pattern may be caused to occur so continuously and change so easily that accuracy in spatial reproduction of an image becomes lowered.
It is to be noted, however, that an optical reading apparatus of a line sensor system of contact type for use in an image scanner for general use is suitable for a reading system having a high degree of accuracy in dimension and resolution. For the optical reading apparatus of such a line sensor system, there is employed a CCD line sensor which, however, requires illuminance as high as approximately 500 to 1,000 lx on a surface to be read. Hence, for scanners for scanning and reading documents and images for general use, a reading surface is illuminated with a light source such as a fluorescent light or the like to provide a sufficiently high degree of illuminance.
In order for the optical reading apparatus of the line sensor system of contact type to read chemical luminescence as high as approximately 10.sup.-8 lx, however, the CCD line sensor is required to be cooled to very low temperatures or it requires an image intensifier, like the CCD image pick-up elements for the camera as described hereinabove. Thus, the optical reading apparatus presents the problems that an optical detector constituting a light receiving portion becomes expensive, the line sensor becomes large in size because it is incorporated into a carriage disposed to be movable in the reading surface, and circulation of a cooling medium is difficult when the optical reading apparatus is of the cooling type.